Rolling plants are known, with a Steckel reversing rolling train with one or more stands, which use a slab with a thickness from 150 to 250 mm or more, and work in coil-to-coil mode, that is, with a slab length that in relation to the thickness is equal in weight to a coil of finished product. In such plants there is a limited surface and dimensional quality of the strip and a limited final thickness, which in general is never less than 1.8-1.6 mm, and in any case such thicknesses are obtained only with great difficulty: the surface quality is limited by the considerable scale that forms during the high number of inversions and passes through the stand/stands, and the connected down times, and by the scale which remains impressed on the final product; the dimensional quality is limited by the great difference in temperature between the leading/tail end and the central part of the strip, and the minimum final thickness is limited by the great thickness of the slab at inlet.
Furthermore, the reversing Steckel mill creates a problem connected to the fact that, in the first rolling passes, the roughed slab (or transfer bar or simply bar) cannot normally be wound immediately in the reel furnaces disposed upstream and downstream of the stand, due to the great thickness of the entering slab, which creates a problem of bulk in the line as the length of the slab increases.
Moreover, the high number of rolling passes, with consequent winding and unwinding in the reel furnaces located upstream and downstream of the stand/stands, induces cooling of the leading and tail ends, and also a lack of uniformity of temperature along the coil, which penalizes the yield due to the need to crop the leading and tail ends.
This large number of passes also causes variable dimensional tolerances along the length of the finished strip and limitations in the production of thin thicknesses; it also causes rapid wear of the work rolls due to the large number of passes and the low temperature of the material being worked and of the leading and tail ends, with a consequent increase in stoppages to change the rolls and therefore a lesser use of the plant.
The entrance of the cold and deformed leading ends into the furnaces upstream and downstream of the stand/stands remains a delicate operation, with risks of jamming that become more and more probable as the thickness of the strip decreases.
WO-A-00/10741 describes a rolling method that, in one form of embodiment, provides a continuous casting step, a roughing step, directly downstream of casting, a heating step carried out after roughing and upstream of a finishing rolling step. In another alternative form of embodiment of WO'741, between the roughing step and the heating step a winding/unwinding step is provided. In another alternative form of embodiment of WO'741, the heating step is the rapid type and is provided directly downstream of casting, whereas the roughing step is provided after the rapid heating, very distant from casting. After the roughing step a winding/unwinding step is provided, after a possible further heating step, which makes the method and connected rolling line according to WO'741 more expensive and dimensionally bigger, and finally the finishing rolling, after which there may follow a possible final pass in the rolling stands (skin pass), to try to obtain the desired final thicknesses.
WO-A-2010/115698 describes a rolling method that only provides a continuous casting step, a roughing step, a rapid heating step after roughing, a step of detecting scale, a pre-cooling step, a de-scaling step and finally a finishing rolling step.
JP-A-59191502 describes a rolling unit provided with a single Steckel type rolling stand, equipped with induction type heating means disposed between the rolls of the rolling stand and the reel furnaces at entrance to and exit from the rolling stand.
The main purpose of the present invention is to obtain a rolling method for flat products and a corresponding line, which can guarantee a final product of high quality in terms of reduced scale impressed, good surface quality and dimensional tolerance even along the length.
Another connected purpose is to obtain an extremely compact plant, having low investment costs and an annual productivity limited from 300,000 to 800,000 tons, which allows to obtain thin strip with a thickness of as little as 1.2 mm or less.
Another purpose of the present invention is to perfect a method that allows to reduce to a minimum the number of rolling passes and inversions, and hence to reduce the total rolling time, with consequent greater uniformity/homogeneity of temperature along the strip being rolled and a lesser overall loss of temperature of the strip.
Another purpose is to increase the factor of use of the plant, increasing the operating duration of the work rolls.
Furthermore, another purpose of the present invention is to exploit to the utmost the high plasticity of the steel at the high temperatures that it possesses when it has just solidified, to carry out the roughing rolling of the product exiting from the continuous casting machine, so that it is thus possible to use smaller stands and hence with a lower power installed and considerable energy saving.
Another purpose is to obtain a continuous casting and rolling method without intermediate storage and recovery of material, and therefore lower heating energy.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.